obsolete fu'ivla x1 emits light characteristic of/is of the color that is described by/(as) arising from/associated with the approximately twenty-one centimeter wavelength, hydrogen hyperfine proton/electron spin-flip transition (from parallel to antiparallel configuration, id est: from the higher energy state to the lower energy state), electromagnetic radiation In analogy to xunre and the like. See also: skari, blabi, xekri, kandi, carmi, nukni, narju, rozgu, zirpu, pelxu, xunre, cidro, lektoni, protoni, dikca, maksi, guska'u, gusni, cradi. This color is a subset/element (depending on interpretation/usage) of the colors associated with light in the radio and microwave regions of the electromagnetic spectrum. Speaker determines how close "approximately twenty-one centimeters" is to exactly twenty-one centimeters. Technically can only be used for the coloration of light that would arise from the hyperfine spin-flip transition of a hydrogen atom (without any neutrons) in which the proton-electron quantum spin configuration abruptly changes from parallel to antiparallel in the 1s groundstate. But other isotopes and/or hydrogenic (id est: single-electron) atoms can be referenced by semantic broadening; note that in any case, this word always refers to the color of the light emitted from such an object. This is a color, not a description of the process, conditions under which the light of this color is emitted, etc., nor is it the light itself nor wavelength of the light. However, this term probably will come up in all such descriptions. For example, "hyperfine spin-flip transition" might be rendered {xipfne binxo}. Technically, any object that emits photons with wavelengths of approximately twenty-one centimeters will be of the color xipfne, regardless of why such emission is occurring (id est: it need not be due to hydrogen hyperfine, proton/electron spin-flip transitions). Usage in such a case is perfectly acceptable. However, in practice, such occasions/contexts will be rare (essentially completely absent except in theory) and the only common usage will be in the context of hydrogen hyperfine proton/electron spin-flip transitions (as in astronomy, chemistry, or quantum mechanics). This is an electromagnetic (id est: light, photon) color. It is physical (being derived from the properties of the wavelength of the emitted and received photon(s)), but is interpreted by some instrument (such as a telescope/camera system, an animal's optical system, etc.). Usage need not be technical.